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BioOne sees sustainable scholarly publishing as an inherently collaborative enterprise connecting authors, nonprofit publishers, academic institutions, research
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Effects of forest composition on trophic relationships among mast production and
mammals in central hardwood forest
Author(s): Carolyn A. Gillen and Eric C. Hellgren
Source: Journal of Mammalogy, 94(2):417-426. 2013.
Published By: American Society of Mammalogists
DOI: http://dx.doi.org/10.1644/12-MAMM-A-138.1
URL: http://www.bioone.org/doi/full/10.1644/12-MAMM-A-138.1
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Journal of Mammalogy, 94(2):417426, 2013
Effects of forest composition on trophic relationships among mastproduction and mammals in central hardwood forest
CAROLYN A. GILLEN AND ERIC C. HELLGREN*
Cooperative Wildlife Research Laboratory, Department of Zoology, and Center for Ecology, Mailcode 6504, Southern
Illinois University, Carbondale, IL 62901-6504, USA
* Correspondent: hellgren@siu.edu
Oak-dominated forest has declined in the eastern United States as shade-tolerant mesophytic species (e.g., maple
[Acer spp.]) replace oaks (Quercus spp.), sparking concern among ecologists regarding species that consume
acorns. Our goal was to describe how increasing mesophication of oak forests may affect consumers in higher
trophic levels. We investigated relationships among forest composition, mast production, small-mammal density,
and carnivore occurrence in 8 stands representing a gradient of oakhickory dominance in central hardwood
forest in southern Illinois. We livetrapped small mammals for.24,000 trap-nights in JuneAugust 20092011
with trapping webs to estimate population density of mice (Peromyscus spp.). We collected mast seeds during
OctoberNovember 20092010 and calculated average dry biomass (g/m2) for each species and stand. During
winters 20092011, we photographed carnivores using baited camera traps. We regressed mast biomass on
measures of forest composition and regressed Peromyscus density and carnivore occurrence on estimates of mast
biomass. Peromyscus summer density was not related to percent hard-mast basal area or hard-mast biomass from
the previous autumn. Logistic regressions of carnivore occurrence on Peromyscus density were not significant.
Many other studies have demonstrated links of several species to oak forest and mast production, but the lack of
associations that we observed was consistent with recent meta-analyses across latitude. The landscape matrix of
oakhickory forest and alternative soft-mast foods also may act to homogenize Peromyscus density across our
study sites. Maintenance of stand heterogeneity in the forest landscape will support a wider diversity of species.
Key words: acorns, mesocarnivores, mesophication, Peromyscus, Quercus, trapping webs
2013 American Society of Mammalogists
DOI: 10.1644/12-MAMM-A-138.1
Recent decades have seen a decline in the distribution of
oak-dominated forests in the eastern United States due to
reduced oak regeneration (Abrams 1998; Aldrich et al. 2005;
Lorimer 1984). Shade-tolerant species, especially red maple
(Acer rubrum), sugar maple (A. saccharum), and American
beech (Fagus grandifolia), are better competitors in light-
limited conditions and are replacing oaks (Quercus spp.) in the
understories of these forests. Nowacki and Abrams (2008,
p.123) label this process mesophication and define it as a
positive feedback cycle . . . whereby microenvironmental con-
ditions (cool, damp, and shaded conditions; less flammable fuel
beds) continually improve for shade-tolerant mesophytic
species and deteriorate for shade-intolerant, fire-adapted
species.
Several interacting factors have been hypothesized for the
change in oak forest dynamics in eastern North America
(McEwan et al. 2011). Fire suppression for much of the 20th
century allowed fire-sensitive maples to gain a foothold in the
understories of eastern oak forests (Abrams 1992; Lorimer
1984). The removal of livestock grazing pressure also acted to
release understory seedlings and increase stem density of
shade-tolerant species that can outcompete young oaks in light-
limited conditions (Aldrich et al. 2005; Spetich and Parker
1998). Uneven-aged timber harvest techniques may suppress
oak regeneration by mimicking gap succession, a disturbance
regime common to late-successional forests (McDonald et al.
2008b; Ozier et al. 2006). Insects, disease, and herbivory by
deer also have contributed to oak decline (Aldrich et al. 2005;
Beals et al. 1960; Oak 2002; Rooney and Waller 2003).
Extinction of the passenger pigeon (Ectopistes migratorius)
removed an important disturbance mechanism that opened the
canopy, released oaks in the understory, and may have
contributed to dominance of oakhickory forest in the early
20th century (Albrecht and McCarthy 2006; Ellsworth and
w w w . m a m m a l o g y . o r g
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McComb 2003). Finally, climate variation over the past 500
years and land-use dynamics also may have contributed to the
species composition shift (McEwan et al. 2011).
Abundant mast production in oak forests can cause
interactions and cascading effects through multiple trophic
levels, involving primary and secondary consumers, parasites,
and disease (Clotfelter et al. 2007; Jones et al. 1998; Ostfeld et
al. 1996; Schmidt and Ostfeld 2003; Fig. 1). Therefore, forestcompositional changes associated with mesophication may
affect a variety of consumers, especially because acorn mast is
considered a keystone resource in oak forests (Clotfelter et al.
2007; Ostfeld et al. 1996). Acorns are highly nutritious and
decompose slowly (Rodewald 2003), providing an important
food source during fall and winter for a variety of mammalian
and avian species (DeGange et al. 1989; Eiler et al. 1989;
Martin et al. 1961; McShea and Schwede 1993). However, the
pulsed nature of acorn production can affect consumers. Many
rodent species exhibit population irruptions in the spring and
summer following a heavy mast crop in the fall via winter
reproduction (King 1983; McShea 2000; McShea and Schwede
1993; Ostfeld et al. 1996; Wang et al. 2009; Wolff 1996). Alarge acorn crop can increase survival of rodents by improving
body condition and reducing vulnerability to predators through
reduced foraging time (Ostfeld 2002).
The loss of acorns in the transition of oak forests to maple
could potentially cause bottom-up trophic effects on both
primary and secondary consumers in the community, yet links
between primary and secondary consumers in relation to acorn
production has not been widely studied. In the absence of
periodic bumper acorn crops, carrying capacity of primary
consumers (e.g., mice) may be reduced, lowering abundance or
density in some areas. Secondary consumers (e.g., mesocarni-
vores) are highly mobile and should move to areas with enough
resources to support them (i.e., areas of high prey density
Chamberlain and Leopold 2000; Litvaitis and Shaw 1980).
Occurrence patterns (presenceabsence) of mesocarnivores in
eastern forest (coyotes [Canis latrans], bobcats [Lynx rufus],
gray foxes [Urocyon cinereoargenteus], and red foxes [Vulpes
vulpes]) probably reflect local prey density sooner thanpopulation size, due to high vagility. Shifts in forest
composition will likely result in altered occurrence and relative
abundance of both groups of consumers along an oak-
dominance gradient from drier, fire-adapted communities to
mesophytic forest. However, there is a notable lack of data
addressing the effects of mesophication on faunal assemblages
(Rodewald 2003).
We initiated testing of the hypothesis that increasing
mesophication of oak forests affects abundance of primary
consumers (i.e., rodents) and occurrence of mesocarnivores
through bottom-up trophic processes. Overall, we predicted
positive relationships among oakhickory dominance, mast
production, rodent abundance, and mesocarnivore occupancy
(Fig. 1). We identified the following objectives to test these
predictions across a gradient of oakhickory dominance in
central hardwood forests: quantify variation in mast availabil-
ity; quantify variation in small-mammal abundance, specifical-
ly Peromyscus spp. density; quantify occurrence of 4
mesocarnivore species (C. latrans, L. rufus, U. cinereoargen-
teus, and V. vulpes) across the oakhickory gradient; and
investigate relationships among mast availability, primary
consumer density, and mesocarnivore occurrence.
MATERIALS AND METHODS
Study area.We conducted research in 8 forest stands
located throughout the Ozark Hills in parts of Shawnee
National Forest and Trail of Tears State Forest, centered at
approximately 378300N and 898220W. The Ozark Hills of
southern Illinois, primarily located in Jackson and Union
counties, compose the easternmost edge of the Missouri Ozark
Mountains. Braun (1950) placed the Ozark Hills in the western
mesophytic forest region, a transitional region characterized by
mosaic patterns of dominant species. McNab (2011) recently
defined this area as the Western Dry subregion of the Central
Hardwood region. Oakhickory forest dominates the upland
areas, whereas pockets of mesophytic species can occur in
ravine bottoms or on sheltered slopes (Braun 1950). Overallcomposition of the Shawnee National Forest, which contains
.1 1 0,00 0 h a o f f or e st a c ro ss sou t he rn I ll in oi s, i s
approximately 68% oakhickory, 16% maplebeech, and
12% pine or oakpine (Haugen 2003).
We chose stands across a gradient of oakhickory
dominance (Table 1). Average stand size was 4.7 ha (range:
3.66.9 ha) and stands were separated by at least 300 m. Stands
comprised mature forest types varying due to elevation, slope,
aspect, and disturbance history. No timber harvest has occurred
in any of these stands for at least 20 years, with single-tree
selection being the primary silvicultural method in Trail of
FIG. 1.Conceptual model of relationships among some species or
guilds in oak forests, adapted from Ostfeld et al. (1996). Arrowsindicate the direction of influence between units. Plus and minus signs
indicate a positive or negative relationship between units, respectively.
Bolded, shadowed boxes highlight the elements of interest for this
project.
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Tears prior to 1989 (Ozier et al. 2006). Stands included
bottomland oak-dominated forest with very little maple in the
understory, oak-dominated upland sites with a minor under-
story component of beech and sugar maple, mixed oak
mesophytic forest, and mesophytic, nonoak forest. Bottomlandoak stands were dominated by pin oak (Quercus palustris) and
cherrybark oak (Q. pagoda); upland oak stands were
dominated by white oak (Q. alba), black oak (Q. velutina),
and hickory (Carya spp.); mixed-mesophytic stands were
characterized by a combination of white oak, red oak,
sweetgum (Liquidambar styraciflua), tulip poplar (Lirioden-
dron tulipifera), and sugar maple; and nonoak stands were
dominated by tulip poplar, sweetgum, and sugar maple.
Stand characteristics and mast production.We measured
habitat characteristics within 3 randomly placed 0.04-ha plots
(radius 11.3 m) in each forest stand in May 2009. We
recorded tree species and diameter at breast height (DBH) of all
.2-cm DBH trees, calculated basal area (BA) for trees usingDBH measurements, and determined relative BA by dividing
the BA for each species by the total BA. We also calculated
percent of BA that was composed of hard-masting trees (i.e.,
Quercus spp., Carya spp., Juglans nigra). We calculated
species richness, diversity (H 0), and evenness of trees using the
ShannonWiener index (Horn 1966). We measured stem
density with two 2 3 10-m transects placed randomly
through the center of each plot. All stems ,2 cm DBH were
counted and placed into height classes of 01, 12, and 23 m.
Vines were counted as a part of stem density, but not placed in
height classes. We recorded percent canopy cover using a
densiometer at the center and 4 corners of the stem-density
transects. We collected 4 readings at each of these 5 within-plotlocations, and averaged them.
We collected mast on the ground from sixty 1-m2 plots at
each study stand during OctoberNovember 2009, and from 50
plots in OctoberNovember 2010. We randomly placed mast
plots along transects placed within each study stand. We
searched through the layer of leaf litter to bare ground,
collecting any mast seeds found in this stratum. We sorted
acorns and other mast seeds by species and grouped hickory
nuts by genus. Acorns were identified by shape, size, and cap
characteristics. We assumed that acorns without caps that were
difficult to identify were the same species of oak as the
majority of loose caps collected at the plot. We recorded the
initial mass using an electronic balance for seeds of each
species, as well as the number of acorns and acorn caps
collected for each plot. Caps still attached to acorns were
broken off and discarded to avoid double counting of caps. Werandomly selected mast from 20 plots from each site. We
sorted seeds by species, placed them in paper bags, recorded
the mass, and dried them at 608C until they reached constant
mass. We then calculated dry biomass estimates for each
species in grams per square meter, grouping mast species into
hard and soft mast.
Small-mammal density.We livetrapped small mammals
during JuneAugust 20092011. We arranged Sherman live
traps (7.53 9.03 23.0 cm; H. B. Sherman Traps, Tallahassee,
Florida) in a trapping-web design (Parmenter et al. 2003)
consisting of 12 transect lines 60 m in length radiating out from
a center point every 308. Traps were placed along each line at
5, 10, 15, 20, 30, 40, 50, and 60 m from the center point,producing 8 trap rings in the web. Additionally, 4 traps were
placed at the center of each web, resulting in 100 traps per web.
Traps were opened in late afternoon (approximately 1500
1700 h) and baited with rolled oats. We also set 24 Tomahawk
traps (16.5 3 16.5 3 48.0 cm; Tomahawk Live Trap Co.,
Tomahawk, Wisconsin) per site in 2009 and 12 Tomahawk
traps per site in 2010, targeting tree squirrels. We baited these
traps with walnuts and peanut butter. We conducted trapping at
each site using a robust design (Pollock 1982) of 3 monthly
sessions consisting of 1 night of prebaiting and 4 consecutive
nights of trapping. We checked traps in the morning before
1000 h and closed them during the day. We identified rodents
and shrews to species and examined them for mass, sex, hind-foot length, and reproductive condition before marking them
with unique toe clips and releasing them at the appropriate trap
stations. Capture and handling of animals followed guidelines
of the American Society of Mammalogists (Sikes et al. 2011)
and were conducted under permits approved by the Southern
Illinois University Carbondale Institutional Animal Care and
Use Committee (permit 09013). Trapping effort with
Sherman traps was calculated using the method suggested by
Beauvais and Buskirk (1999), wherein trapping effort is
adjusted by multiplying all sprung traps, including both
animal captures and accidentally sprung traps, by 0.5. This
TABLE 1.Tree and mast characteristics of 8 hardwood forest stands in southern Illinois, 20092010. Standard errors for mast production are
given in parentheses.
Stand
size (ha)
Hard-mast
basal area (%)
Total basal
area (m2/ha)
Species
diversity (H0)
Species
richness
2009 mast (g/m2) 2010 mast (g/m2)
Hard
Soft
Hard
SoftAcorn Other Acorn Other
6.9 2.9 51.41 1.14 19 0.03 (0.03) 0.12 (0.08) 5.63 (1.17) 0.21 (0.06) 0.48 (0.07) 14.47 (1.15)
4.1 8.7 34.88 1.44 20 0.09 (0.04) 1.66 (1.00) 5.63 (1.40) 1.08 (0.33) 3.16 (1.05) 10.71 (0.63)
5.2 21.29 34.27 1.57 22 0.71 (0.42) 4.44 (1.85) 7.25 (1.46) 1.66 (0.22) 2.59 (0.88) 11.25 (1.00)
4.2 46.95 37.43 1.34 15 29.02 (4.02) 0.00 6.47 (1.25) 36.35 (3.40) 0.00 5.88 (1.31)
5.0 53.34 40.32 1.69 10 0.72 (0.22) 36.86 (8.21) 1.47 (1.19) 1.47 (0.43) 22.80 (4.71) 6.62 (1.34)
4.2 84.77 26.12 1.99 20 19.34 (4.45) 7.07 (2.52) 0.03 (0.01) 6.89 (0.52) 7.74 (3.42) 0.34 (0.18)
4.5 90.39 33.97 1.47 14 13.20 (4.30) 1 0.22 (3.46) 0 .04 (0.02) 5.44 (0.68) 1 3.41 (6.42) 0.30 (0.02)
3.6 94.75 36.94 0.50 11 4.87 (0.52) 0.00 0.04 (0.04) 35.91 (4.34) 0.06 (0.03) 0.54 (0.12)
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method reduces the error associated with variation in trap-
springing among sites (Beauvais and Buskirk 1999).
We used program Distance 6.0 (Thomas et al. 2010) to
estimate population density for each site. We combined new
captures ofPeromyscus leucopus and P. maniculatus to obtain
density estimates for the genus. These 2 species are
ecologically very similar (Klein 1960) and were difficult todifferentiate in the field by tail coloration. They composed the
majority of total small-mammal captures each year. We
analyzed 2009 data for each month with the uniform and
half-normal key functions with cosine, polynomial, and
hermite polynomial series expansions and varying orders of
adjustment. Capture data were placed into distance categories
(i.e., bins) specified to include the trap ring distance as the
midpoint of each bin (see trapping-web description above). The
most distant bin ranged from 55 to 65 m to include area outside
the actual web that was likely to contribute to Peromyscus
captures. We chose best models based on the lowest Akaike
information criterion for small sample sizes (AICc) value;
however, if!2 models were within 2 AICc points, we chose
the most precise model (i.e., lowest coefficient of variation
[CV]). We chose a specific model for each site and each month
rather than use a common model for pooled data because
individual models provided lower CVs and narrower confi-
dence intervals for density estimates. Finally, we combined the
2 distance intervals nearest the center point (02.5 m and 2.5
7.5 m) to obtain a bettershoulder near the center point in the
detection probability curve, which helps satisfy the assumption
that detection probability is 1.0 at the web center (Buckland et
al. 2003). Monthly density estimates were averaged to obtain
estimates of summer density for each site.
Carnivore occurrence.We estimated mesocarnivoreoccurrence at our sites using photographs taken by
Cuddeback Excite remotely triggered cameras sensitive to
infrared motion (Non Typical, Inc., Green Bay, Wisconsin).
We placed 1 camera per stand during February 20092010 and
January 2011, positioning each camera 24 m away from a bait
station consisting of a fatty acid scent disk (Department of
Agriculture Pocatello Supply Depot, Pocatello, Idaho) and a
can of sardines attached to a tree. We returned to each site once
per week to change the bait and to collect photos taken during
the previous week. This schedule resulted in three 1-week
sampling periods for each site per month, allowing us to
estimate detection probability (p) and occupancy (w) for each
species (MacKenzie et al. 2002).We used the multiseason model in program PRESENCE 3.0
(Hines 2010) to estimate detection probability and occupancy
of gray fox, coyote, and bobcat at the 8 stands during February
20092010 and January 2011. We recognized the limitations in
scale and sample size associated with our 8 intensively sampled
plots relative to assessing occupancy of carnivores. Therefore,
we added occupancy data collected from cameras located in 30
additional hardwood stands in the Ozark Hills (Jackson and
Union counties) during JanuaryApril 2010 during a large-
scale carnivore survey (Nielsen et al. 2011) to those collected
in our 8 study stands. These additional stands were located in
2.6-km2 sections containing !50% forest cover. Stands were
sampled at survey points using a 103 BA-factor prism to
determine tree composition and BA. Although we did not have
small-mammal data from these additional 30 stands, this
analysis allowed us to assess relationships between forest stand
composition and carnivore occurrence at a suitable spatial
scale. We built models including month as a survey covariateto assess whether p differed between JanuaryFebruary and
MarchApril. In the set of 38 stands, we tested null models and
models varying w by percent hard-mast BA for gray fox,
coyote, and bobcat. Model sets were ranked by AICc.
Trophic analyses.We performed linear and logistic
regression to assess relationships among forest composition
measures, mast production, Peromyscus density, and carnivore
occurrence. Independent variables included hard-mast BA (%),
hard- and soft-mast biomass (g/m2), and Peromyscus average
summer density (N/ha), as well as a suite of forest composition
characteristics. Regressors included hard- and soft-mast
biomass (g/m2), and Peromyscus average summer density (N/
ha). The experimental unit in all cases was the forest stand. Log
or square-root transformations were applied to 1 or both
variables in some cases to obtain a better fit and to satisfy
statistical assumptions. Linear regression code specified AICcvalues for each regression, by which we could later rank
contribution of individual variables to variance explanation. To
prevent comparison of highly correlated variables, we
performed correlation analyses of regressors with Bonferroni
corrections and excluded the lower-ranked variable in a pair of
highly correlated regressors. For all analyses, significance was
defined as a 0.05.
Logistic regressions regressed binary carnivore occurrence
data (1 detected, 0 not detected) against independentvariables including hard-mast BA (%), hard- and soft-mast
biomass (g/m2), acorn biomass, and average summer Peromy-
scus density. Additionally, we included carnivore occurrence
data collected from the 30 additional stands in the concomitant,
large-scale carnivore survey with data collected at the 8 main
sites and regressed these against hard-mast BA (%).
RESULTS
Mast production and mammal characteristics of forest
stands.Stand estimates (n 8) of percent hard-mast BA
ranged along a gradient from 3% to 95%, with an increasing
gradient of hard-mast production and a decreasing gradient ofsoft-mast production across stands (Table 1; Fig. 2). We
collected seeds of 14 species of hard mast, including the genus
Carya, and 7 species of soft mast, including a group of assorted
berries, for a total of 21 mast species in 2009. We collected 20
different species of mast seeds, including Carya spp. and
assorted berries in 2010. Six species of acorns were collected,
including 2 in the white oak group and 4 in the red oak group.
Other species of hard mast were nuts of hickory, black walnut,
and American beech. Acorns and hickory nuts composed the
majority of hard mast collected (Table 1). Hickory nuts were
most abundant at stands falling in the middle of the gradient of
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percent hard-mast BA (Table 1). The soft-mast group included
11 species, such as sweetgum, tulip poplar, sugar maple, and
ash (Fraxinus spp.)
Total effort with Sherman traps in 2009, 2010, and 2011 for
small mammals was 8,050, 8,186, and 7,871 trap-nights,
respectively, after adjusting for sprung traps at each site.
Overall trapping success (total summer captures/adjustedsummer trap-nights) ranged from 6.9% to 8.1% annually, with
P. leucopus and P. maniculatus composing 87.396.5% of
total small-mammal captures. Other species caught during the
study in Sherman traps included southern short-tailed shrew
(Blarina carolinensis), woodland vole (Microtus pinetorum),
golden mouse (Ochrotomys nuttalli), marsh rice rat (Oryzomys
palustris), eastern woodrat (Neotoma floridana), and eastern
chipmunk (Tamias striatus); however, we did not capture
adequate numbers of these species to estimate population
density. We captured 0 gray squirrels (Sciurus carolinensis) in
2,304 trap-nights in 2009 and 4 gray squirrels in 1,152 trap-
nights in 2010 using larger Tomahawk traps; we did not target
squirrels in 2011. We estimated Peromyscus density for each
month at all sites except for 2 sitemonth combinations in 2009
and 1 site in 2010 due to extremely low capture success. The
range of average density estimates was 0.88.5 mice/ha in
2009, 0.124.5 mice/ha in 2010, and 1.514.9 mice/ha in 2011
(Fig. 3).
We detected each of the 4 targeted mesocarnivore species at
least once during February 2009, all but the red fox during
February 2010, and only coyotes and bobcats in January 2011.
Gray fox occupancy was estimated to be 0.65 (SE 0.38) with
a detection probability of 0.25 (SE 0.13) across all 8 sites.
Limited data allowed parameter estimation only when
occupancy and detection probability were held constant
(w(.)p(.)). Parameters could not be estimated for coyotes or
bobcats using only the original 8 stands; however, we obtained
estimates after including data from 30 additional stands. For
coyotes, the top-ranked model held p constant and allowed w to
vary by percent hard-mast BA (Table 2). A model testing for
differences in detectability by survey month showed no
difference in p between JanuaryFebruary (0.52 6 0.14 SE)
and MarchApril (0.49 6 0.09). Conditional occupancy of
coyotes (i.e., probability of occupancy given detection history)tended to decrease with increasing percent hard-mast BA, with
average w 0.63 (SE 0.10). The top-ranked bobcat model
held both w and p constant (Table 2). Detection probability was
0.15 (SE 0.09) across sites, and average w was 0.82 (SE
0.47).
Trophic relationships.Not surprisingly, hard- and soft-
mast production showed clear relationships with percent hard-
mast BA (Fig. 2). Hard-mast biomass (g/m2) showed a positive
relationship with percent hard-mast BA in 2010 (F1,6 9.67, P
0.02, r20.62), and trended toward a positive relationship in
2009 (F1,6 4.84, P 0.07, r2 0.45; Fig. 2). Similarly, the
FIG. 2.Regressions of mast biomass (averaged across 5060 plotsper stand) on hard-mast basal area (%) for a) hard mast and b) soft
mast in 2009 (black circles; solid line) and 2010 (white triangles;
dotted line) at 8 stands in southern Illinois. Hard-mast variables in
both years were natural logtransformed to satisfy statistical
assumptions.
FIG. 3.Relationship between Peromyscus spp. density ( X6 SE)
and hard-mast basal area (%) at 8 forest stands in southern Illinois
during summer 2009 (black circles), 2010 (light gray triangles), and
2011 (dark gray squares). Relationships were nonsignificant in allyears.
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strong negative relationships between soft-mast biomass and
percent hard-mast BA in both 2009 (F1,6 21.98, P 0.003, r2
0.79) and 2010 (F1,6 170.11, P , 0.001, r2 0.97; Fig. 2)
supported our choice of percent hard-mast BA as a predictor of
both hard- and soft-mast production.
Relationships between population parameters of Peromyscus
and measures of forest composition or mast were generallyweak. We found no relationship of mouse density in any year
with percent hard-mast BA (Fig. 3); nor did we see positive
responses of mouse density in 2010 and 2011 to total hard-
mast production from the previous year (2010: F1,6 0.36, P
0.60, r2 0.06; 2011: F1,6 1.66, P 0.24, r2 0.22; Fig. 4).
Similarly, acorn-only production in the previous year was not
related to summer mouse density in 2010 (F1,60.01, P0.93,
r2 0.002) and had a marginally positive relationship (F1,63.73, P 0.10, r2 0.38) in 2011. Likewise, soft-mast
production in 2009 and 2010 did not influence mouse density
in 2010 (F1,6 0.59, P 0.47, r2 0.09) or 2011 (F1,6 0.46,
P 0.52, r2 0.07).
We found no significant relationships between occurrence of
any mesocarnivore in 20092011 and percent hard-mast BA in
the 8 main study sites. Similarly, coyote occurrence and bobcat
occurrence in 2010 and 2011 were not significantly related to
the previous years Peromyscus density, nor to the previous
years hard-mast or soft-mast production. No detections of red
fox in 2010 or 2011, and no detections of gray fox in 2011 and
only 1 in 2010 prevented statistical modeling of relationships.
The analysis of data that included 30 additional sites in the
Ozark Hills region in 2010 resulted in a negative relationship
between coyote occurrence and percent hard-mast BA (v21
4.64, P 0.03). Bobcat occurrence was not related to percent
hard-mast BA in the set of 38 sites (v21 0.05, P 0.83).
There were no red fox detections and only 2 gray fox
detections in 2010, which prevented statistical modeling of
relationships.
DISCUSSION
We focused on the signal associated with a gradient of oakhickory dominance to base our investigation of mesophication
effects on higher trophic levels. Because our study area is in the
midst of mesophication (Fralish and McArdle 2009; Ozier et al.
2006; Zaczek et al. 2002) and we did not have complete
histories of each stand, we could not control for slope position
and aspect along the full gradient from oakhickory to
mesophytic forest. We recognized that topographic and
moisture differences among the stands may have contributed
noise to the mast and mammalian metrics that we measured.
Nevertheless, characteristics of overstory and understory
habitat did not vary among stand types in our study area
(Edmund 2011) with the exception of percent hard-mast BA
(the basis of our gradient) and diversity of understory woodystems (higher in mesophytic stands).
In general, our hypotheses were not supported. We did not
see expected variation in Peromyscus density and carnivore
occurrence among forest types. We also expected to find
positive relationships among hard-mast production, Peromy-
scus density, and carnivore occurrence; however, we found
nonsignificant trends among trophic levels. Estimates of
Peromyscus density on trapping webs at our sites were at the
low end of density ranges reported in the literature using grid-
based sampling, which typically underestimates effective area
sampled and therefore overestimates density. Average densities
TABLE 2.Model-selection results for occupancy (w) and detection probability (p) of coyotes (Canis latrans) and bobcats (Lynx rufus) in 38
stands in the Ozark Hills, southern Illinois, during JanuaryApril 2010. Models were ranked by Akaikes information criterion for small sample
sizes (AICc). K is the number of parameters estimated.
Species Modela AICc DAICc AICc weight K Deviance
Coyote w(HMBA)p(.) 134.13 0.00 0.86 3 127.42
Coyote w(.)p(.) 138.33 4.20 0.11 2 133.99
Coyote w(.)p(Apr) 140.66 6.53 0.03 3 133.95Bobcat w(.)p(.) 89.15 0.00 0.57 2 84.81
Bobcat w(HMBA)p(.) 91.00 1.85 0.23 3 84.29
Bobcat w(.)p(Apr) 91.29 2.14 0.20 3 84.58
a HMBA percent hard-mast basal area.
FIG. 4.Relationship of Peromyscus spp. density ( X6 SE) during
summer 2010 (black circles, solid line) and 2011 (gray triangles,
dashed line) with hard-mast production from the previous autumn in 8
forest stands in southern Illinois. Relationships were nonsignificant.
2010: density (0.13*hard mast) 7.56; F1,6 0.36, P 0.60, r2
0.06. 2011: density (0.14*hard mast) 3.07; F1,6 1.66, P 0.24,
r2 0.22.
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across all sites in our study were 4.1 mice/ha in 2009 and 5.5
mice/ha in 2010 and 2011. In comparison, longer-term (!5
years) studies conducted in the eastern United States have
recorded densities of Peromyscus ranging from 0 to .100
mice/ha (Clotfelter et al. 2007; Ostfeld et al. 1996; Wolff
1996). A study in the Missouri Ozarks estimated Peromyscus
densities to range from about 5 mice/ha to about 23 mice/ha(Fantz and Renken 2005). The lack of relationship between
Peromyscus density and percent hard-mast BA in our study
points to unmeasured variation in the system.
The majority of the literature relating mast to small
mammals has documented positive correlations between acorn
mast and the following years small-mammal populations
(Clotfelter et al. 2007; McShea 2000; Ostfeld et al. 1996; Wolff
1996). Our estimates of hard-mast biomass compared reason-
ably well to estimates of acorn production in Virginia and
North Carolina (Diamond et al. 2000; McShea 2000). Diamond
et al. (2000) estimated total hard mast to be 28.0 g/m2; 5 of our
8 stands produced .23 g/m2 in at least 1 year of this study.
McShea (2000) collected acorns using stationary mast traps
and defined a bumper acorn crop to be .30 g/m2 and a mast
failure to be ,5 g/m2. By these terms, hard-mast production in
our study ranged from mast failure to bumper crops across the
stands over the 2 years of sampling. However, despite hard-
mast biomass estimates that spanned widely, we did not see the
commonly observed relationship between total hard mast or
acorn mast and mouse density.
We also found no relationship between Peromyscus density
and soft-mast production in the previous year. This result
contrasted with evidence from northern hardwood forests
dominated by maplebeech communities, which indicated that
soft mast can influence small-mammal abundance (Falls et al.2007; Jensen et al. 2012; McCracken et al. 1999). Although the
magnitude of population growth of P. leucopus from spring to
summer was related to the spring crop of red maple mast,
summer densities were only related to the previous years acorn
crop (McCracken et al. 1999). Martin et al. (1961) recorded
some use of soft-mast by small mammals, but ranked oak mast
above maple and tulip poplar mast in the diet of P. leucopus.
Peromyscus populations in southern Illinois may not exhibit
the dramatic boom-and-bust cycles after mast years commonly
seen in the northeastern United States. In a meta-analysis of
population cycles in P. leucopus in North America, Wang et al.
(2009) reanalyzed 6 long-term data sets ranging in length from
14 to 32 years. They noted a spatial cline in which cycles of P.leucopus were muted, exhibiting weaker direct and indirect
density-dependence with decreasing winter severity and
latitude. They proposed that a more diverse food base in
southern sites may uncouple dynamics of P. leucopus from
acorn production. Our data on considerable soft-mast produc-
tion in stands with few oaks and hickories (Fig. 2), coupled
with the lack of a mouse response to hard-mast variability,
supports their conjecture. Alternatively, the timescale of our
study may have been too short to detect a trend in Peromyscus
density due to high variation between years. Long-term study
(i.e., decades) could be more informative in determining the
relationship between Peromyscus density and hard mast in our
study region.
The relationship between Peromyscus density and hard-mast
production in southern Illinois forests could be muted by
variability in insect assemblages based on forest type. Many
species of small mammals, including P. leucopus and P.
maniculatus, are omnivorous and consume insects (Ostfeld etal. 1996; Semel and Andersen 1988; Whitaker 1963). The
literature documents variability in insect assemblages based on
forest composition; however, much of this research supports
positive links between oak forests and invertebrates, which
would have additional positive effects on higher trophic levels
(Butler and Strazanac 2000; Martel and Mauffette 1997;
Summerville et al. 2008). Further research on how insect
communities may differ along a gradient of oakhickory cover
and how invertebrate prey contribute to small-mammal diets
should help clarify this relationship.
The landscape matrix of oakhickory forest in which our
study sites were located, in combination with the spatial scale
of the stands, also may explain the lack of variation observed in
Peromyscus density among forest types. Forest stands
dominated by alternate species were relatively small and
surrounded by the larger oakhickory matrix; therefore, rodent
communities living in nonoak or mixed mesophytic stands may
be influenced by oakhickory dominance. Although our stands
were a magnitude larger than mice home-range sizes, mice will
quickly occupy available space (Schmidt et al. 2001).
Therefore, mice may move, for example, from upland oak
forest into mesophytic, nonoak stands. Estimates of immigra-
tion and emigration rates for mice in individual stands were
high (often .0.50Gillen 2011), implying that individuals
living on the edges of stands may cross borders andhomogenize mouse densities across forest types.
We did not observe a significant relationship between
carnivore occurrence and Peromyscus density at our 8 sites. An
important factor influencing carnivore occurrence is the
distribution of prey (Chamberlain and Leopold 2000; Litvaitis
and Shaw 1980). If carnivores move to areas with enough
resources to support them (i.e., areas of high prey density), then
carnivore occurrence should be related to prey density.
However, in our study, the range of mouse densities may
have been too narrow (Fig. 3), and the vagility of the
mesocarnivores too great (Litvaitis and Shaw 1980; Major and
Sherburne 1987; McDonald et al. 2008a) relative to the scale of
our stands to detect the pattern. In addition, the availability ofalternative prey (e.g., arthropods and birds) was unknown.
The negative relationship between coyote occurrence and
percent hard-mast BA in the larger sample of sites (n38) was
strong, but is not easily explained, especially with the lack of
correlation between percent hard-mast BA and Peromyscus
density seen at the 8 main study sites. Our sites comprised
different stand types of mature forest with similar BA. Coyotes
are generalists with regard to habitat selection, with large-scale
estimates of abundance being positively correlated with forest
cover, especially disturbed forest and natural edges, such as
along wetland zones (Kays et al. 2008). Coyotes could be
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exploiting resources that occur more frequently in mesic
nonoak forest, such as alternative prey or den sites. Mesophytic
stands in our study area were located in valleys drained by
intermittent creeks that had potential to provide good cover for
den sites (Hallett et al. 1985).
Mesophication has been widely assumed to be detrimental to
a diversity of terrestrial vertebrates that depend on oaks forfood (Aldrich et al. 2005; Fralish and McArdle 2009;
Rodewald and Abrams 2002; Summerville et al. 2008);
however, the magnitude of the potential effect has not been
widely studied. Examination of our data, which contain a lack
of significant relationships among hard-mast biomass or
percent hard-mast BA, Peromyscus density, and mesocarnivore
occurrence, suggests a less extreme view of the consequences
of mesophication, at least in these forest stands in southern
Illinois. As mentioned above, more diverse foods in southern
hardwood forests may reduce the links between acorn
production and Peromyscus dynamics (Wang et al. 2009).
Furthermore, McShea et al. (2003) found that mesic forestpatches in the southern Appalachians contained higher
abundances of several species of small mammals than nearby
xeric patches. Mice are generalist consumers that take
advantage of abundant acorn crops when they occur, but can
persist during years of mast failure (Clotfelter et al. 2007;
McShea 2000; Wolff 1996). In the absence of oaks, the small-
mammal boom-and-bust cycles associated with acorn mast
years may disappear, but populations could potentially stabilize
somewhat as small mammals take advantage of soft mast that
provides more stable sources of food. Population increases due
to bumper crops of soft mast, such as maple or ash (Jensen et
al. 2012), may become more pronounced. Although generalist
species, such as P. leucopus and P. maniculatus, will probably
adapt to the loss of oak forest, some specialist species will no
doubt be more negatively affected. It is also possible that
effects may be felt more keenly by consumers in higher trophic
levels.
If oakhickory forests transition to maplebeech forests,
beechnuts and maple mast could mitigate the loss of acorns to
some extent, because their production can vary over time in a
manner and magnitude similar to acorn production (Jensen et
al. 2012; Overgaard et al. 2007). Indeed, long-term data from
northern beechmaple forests showed strong links among
beech and maple mast, small mammals, and 2 mustelid
predators (Jensen et al. 2012) parallel to those modeled in oakforests (Fig. 1; Ostfeld et al. 1996). Based on average dry mass
of a beechnut (0.1 g) and reports of nut abundances (Leak and
Graber 1993; McNulty and Masters 2004; Rosemier and Storer
2010), we estimated peak beechnut production to be 2.319.8
g/m2 in northern maplebeech forests. Jensen et al. (2012)
reported peak beechnut production to be 416 g/m2 based on
seedfall traps. These estimates are within the bounds of total
hard-mast biomass in oakhickory stands from this study and
from Diamond et al. (2000), and can be supplemented by
equivalent or greater amounts of soft mast from Acer spp.
(Jensen et al. 2012).
The process of mesophication has been occurring and will
continue for several decades, likely altering mammalian and
other wildlife assemblages (Rodewald 2003). Examination of
our data showed no clear links between a gradient of oak
hickory dominance and Peromyscus density, suggesting that
the consequences of mesophication may not be as severe for
mice as for other species (Rodewald 2003) in the CentralHardwood region. Mammal assemblages in eastern forests
have already dealt with the loss of a keystone resource through
the demise of the American chestnut (Castanea dentata) in the
early 20th century. Chestnut crops did not fluctuate as
dramatically as acorn crops and therefore provided a more
stable source of food (Diamond et al. 2000; Steele et al. 2005).
We predict that Peromyscus populations would successfully
adapt to changing forest conditions, allowing conservation
efforts to be focused on other species or guilds likely to be
detrimentally affected by mesophication. Accordingly, we
recommend maintenance of forest stand diversity and hetero-
geneity (e.g., a mixture of oakhickory and mesophytic forest
patches) in the landscape to support a wider diversity of species(McShea et al. 2003; Sabo et al. 2005).
ACKNOWLEDGMENTS
Funding was provided by the Illinois Department of Natural
Resources through project W-135-R. We thank Shawnee National
Forest and Trail of Tears State Forest for allowing us to conduct
research in these areas. We also appreciate the technical assistance of
N. Alexander, A. C. Edmund, D. B. Lesmeister, L. Manuszak, S.
Ramakrishnan, and many volunteers. We thank E. M. Schauber and C.
Ruffner for editing and advice, and W. P. Smith and 2 anonymous
reviewers for helpful and constructive comments.
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